The multi-antenna technology is an important component of the fourth generation wireless communication system. An important application in the multi-antenna technology is closed-loop downlink pre-coding. In this application, when signals are sent to a client by multiple antennas located in a base station, the client measures a downlink transmission channel from the base station to the client, and suggests a corresponding pre-coding matrix to the base station according to the channel condition. This suggestion is transmitted to the base station via a feedback link. Subsequently, in the downlink transmission, the base station may adopt directly the suggested pre-coding matrix to pre-code the signal to be sent to the client.
In the above procedure for closed-loop downlink pre-coding, an important issue is how to feed back the pre-coding matrix suggested by the client. The means which is most popular and possibly most practical is a manner of codebook based feedback. In such a feedback manner, a codebook containing multiple pre-coding matrices is first defined and is applied at both the base station and the client. In this way, when suggesting a certain pre-coding matrix in the codebook to the base station according to the channel condition, the client can feed back the index corresponding to the pre-coding matrix to the base station. For instance, a codebook containing four pre-coding matrices, each of which is indexed respectively as {00}, {01}, {10} and {11}, is known at both the base station and the client. If the client suggests using the first pre-coding matrix, the bits {00} are fed back to the base station. A feedback manner based on the pre-coding matrix index can achieve improvement in both reducing the feedback overhead and assuring a high downlink throughput.
A basic codebook design is based on feedback of a pre-coding matrix for a single antenna array. However, in the current LTE-A (Long Term Evolution-Advanced) standardization, a client may receive signals from multiple antenna arrays located in multiple base stations, for example, in downlink CoMP (Coordinate Multiple Points) as shown in FIG. 1. In such a case, a suggested pre-coding matrix needs to be fed back for each antenna array, and thus multiple pre-coding matrices need to be fed back for the multiple antenna arrays. In addition, in order to enable signals from the multiple a) antenna arrays be combined coherently at the client, the phase offsets between the pre-coding matrices need to be fed back to the base station as well.
With respect to the feedback manner of the pre-coding matrices under the above downlink CoMP environment, for simplification, as an example, assuming that the base station 1 and the base station 2 are involved in the downlink transmission, and each of the base stations uses an antenna array containing two antennas for transmitting signals (as shown FIG. 2), the description of a currently known feedback manner is made in the following.
For the downlink CoMP environment as shown in FIG. 2, a manner of direct feedback at present is to use a two-antenna codebook defined in LTE standard Rel-8 version for both the base station 1 and the base station 2, i.e.
      [                            1                          1                          1                          1                                      1                                      -            1                                    j                                      -            j                                ]    /            2        .  There are four pre-coding matrices
      [                            1                                      1                      ]    ,      [                            1                                                  -            1                                ]    ,            [                                    1                                                j                              ]        ⁢                  ⁢          and      ⁢                          [                                    1                                                              -              j                                          ]      included in the two-antenna codebook. Therefore, when the client performs feedback, two bits are used to feed back PMI1 (Pre-coding Matrix Index) suggested to be employed for the antenna array located in the base station 1, and two bits are used to feed back PMI2 suggested to be employed for the antenna array located in the base station 2 as well. Furthermore, one bit is additionally required to feed back the phase offset (−1 or 1) between the PMI1 and the PMI2 to the base station.
In the above feedback manner, five bits in total are used for feedback. In some cases, however, people may expect to reduce the feedback overhead. In such cases, two possible feedback manners are known as follows.
The first possible manner is substantially the same as the above five-bit feedback manner, and the difference is that the one bit for feeding back the phase offset between the PMI1 and the PMI2 is removed, that is, the phase offset is not fed back. In this way, only four bits in total are needed for feedback. However, since the phase offset is not fed back, the signals transmitted from the antenna arrays of the base stations 1 and 2 may not be combined coherently at the client, thus resulting in the reduction of the system throughput.
Another possible manner is to reduce the number of bits for a pre-coding matrix index (for example, PMI2) from two bits to one bit, and meanwhile one bit is still used to feed back the phase offset. Specifically, the two-antenna codebook defined in the LTE standard Rel-8 version, that is
            [                                    1                                1                                1                                1                                                1                                              -              1                                            j                                              -              j                                          ]        /          2        ,still used for the base station 1, while a two-antenna codebook containing two pre-coding matrices
      [                            1                          1                                      1                                      -            1                                ]    /      2  is used for the base station 2. In this way, when the client is feeding back, two bits are still required to feed back the PMI1 for the antenna array located in the base station 1, while only one bit is required to feed back the PMI2 for the antenna array located in the base station 2. In addition, one bit is used to feed back the phase offset between the PMI1 and the PMI2 to the base stations. Thereby, the feedback overhead is reduced to four bits. However, the method results in reduction of the spatial granularity of the codebook of the PMI2, and may also cause reduction of the throughput.
Thus, it has become an issue of the art how to reduce the feedback overhead while keeping the precision of feedback for a multi-antenna array.